Unblock addon versions from follow-up actions on appeal

[eviljeff]

Fixes: mozilla/addons#16119

Description

Adds appeal support for follow-up actions - which are (currently) delayed block actions. Blocked versions are unblocked; pending BlocklistSubmissions are deleted or amended.

Context

In this patch I added a reverse_action to the ContentActionDelayedXxxYyyBlockAddon classes, because I didn't like how ContentActionTargetAppealApprove.process_action was getting longer and longer. The thinking was that we can delegate the reversal of an action to the class that executed it. If it seems like a good model we can rewrite the other actions to work the same way. Though it's not without it's problems - we often want different functionality (e.g. target_versions; emails) for an appeal that reverses an action compared to the initial action.

Follow-up actions are always executed after a primary action. The filtering in the view prevents any of AMO_FU_DELAY_XXX_YYY_BLOCK_ADDON decision actions from being primary actions, but I've attempted to write the action class support so if it was executed as a primary action the appeal would still work.

Testing

• report an add-on
• add or one more enforcement actions to a policy in Cinder
  • e.g. short soft block + mid hard block
• process the job in cinder using that policy
• replay the webhook locally
  • double check the payload that all the enforcement actions are there
  • existing functionality:
    • you will have to 2nd level approve if the add-on is promoted
    • the add-on should be disabled
    • there should be follow-up actions executed - 2 pending (delayed) blocklistsubmissions if you followed the e.g.
• to test all the functionality (i.e. the block deletes too), edit one of the blocklistsubmissions to remove the delayed date and save, so the block is added immediately
  • check that block exists now in django admin
• appeal the decision with the link from the email in admin
• process the appeal in Cinder, overriding the decision, to Approve
• replay the webhook to process the appeal
• the add-on should be re-enabed
• the block should be removed
• the blocksubmission should be deleted

Checklist

• Add #ISSUENUM at the top of your PR to an existing open issue in the mozilla/addons repository.
• Successfully verified the change locally.
• The change is covered by automated tests, or otherwise indicated why doing so is unnecessary/impossible.
• Add before and after screenshots (Only for changes that impact the UI).
• Add or update relevant docs reflecting the changes made.